1. Field of the Invention
The present invention relates to a digital network system having an arrangement for testing a digital subscriber line. The digital network system provides a digital subscriber line testing system for carrying out a transmission test of digital subscriber lines between a subscriber circuit of an exchange station in an integrated services digital network (ISDN) or the like and terminal equipment at customer station lines.
2. Description of the Related Arts
In general, in the integrated services digital network, several kinds of bus type terminal equipment are connected to the terminal equipment of customer station lines, and digital subscriber lines are connected between the terminal equipment of the customer station lines and a subscriber circuit of a subscriber switch.
It is necessary to check these digital subscriber lines to determine whether or not the transmission quality is that desired, as in the case of ordinary transmission lines.
The integrated services digital network (ISDN) is being standardized under the direction of the International Telegraph and Telephone Consultative Committee or CCITT, as described above, and accordingly, the line between a subscriber circuit of a subscriber switch and terminal equipment of customer station lines is connected by a digital subscriber line. As a form of transmission data, for instance, a (2B+D) format is selected that is composed of a channel D of 16 Kb/s and a channel B of 64 Kb/s. A frame synchronization signal bit and a control bit (or a housekeeping bit) are added to the data to comprise one frame. If the control bit is used only for starting the terminal equipment of customer station lines, only one bit will be sufficient for such a control bit. However, where a turn-around loop is formed with this terminal equipment, and a transmission test of the digital subscriber line is carried out, another control bit is necessitated in the prior art to instruct the formation and removal of the above-described turn-around loop, and thus there has been a demand for at least two control bits.
In the digital subscriber line, desirably the effect of crosstalk upon other lines is greatly reduced, relying upon the decline of a transmission bit rate as much as possible, and the transmission line section is lengthened. For this reason, the amount of all control data other than the inherent data must be minimized.
As described before in connection with the prior art digital subscriber line transmissio, there is thought to be a need for at least two control bits, but in accordance with the present invention, it is possible to carry out the formation and elimination of a turn-around loop by means of just one control bit, and to execute a transmission test of the digital subscriber line.